Summary

The flag AliasLevel (HotSpot VM command line flag) is to be deprecated, since its values are broken or redundant since 2015.

Problem

The range for AliasLevel is [0-3] with the following option description:

"0 - for no aliasing, "
"1 - for oop/field/static/array split, "
"2 - for class split, "
"3 - for unique instances"

The default value is 3. Only values 2 and 3 are currently supported. The VM terminated with an error with AliasLevel's value 0 or 1. Value 2 is redundant because the same behavior can be triggered by setting -XX:-EliminateAllocations.

Solution

AliasLevel's value 2 turns off split_unique_types transformation in C2 JIT escape analysis. But split_unique_types is also guarded by the EliminateAllocations flag. So -XX:AliasLevel=2 can be replaced with -XX:-EliminateAllocations.

• Deprecate AliasLevel in JDK 19.
• Obsolete it in JDK 20
• Expire it in JDK 21.

Users will now get the following message when running:

java -XX:AliasLevel= Java HotSpot(TM) 64-Bit Server VM warning: Option AliasLevel was deprecated in version 19.0 and will likely be removed in a future release.

Specification

Deprecate the AliasLevel option in JDK 19 and issue the above warning message when the option is used.